Glycated hemoglobin (glycohemoglobin) in which sugar is bonded to hemoglobin in blood, particularly hemoglobin A1c (hereinafter, referred to as “HbA1c”) in which the N-terminal valine residue of the β-chain of hemoglobin is glycated, clinically reflects the average blood sugar level of the past one to two months. Therefore, glycated hemoglobin, or glycated hemoglobin A1c, is widely used as a marker appropriate for the diagnosis of diabetes or a clinical observation of diabetes.
As the method for measuring glycated hemoglobin, HPLC methods and immunological assay methods are known, but in the immunological assay methods, it is required to use antibodies that are specific to the glycated site of hemoglobin. It has been found that the glycation site at the N-terminal of the β-chain of hemoglobin is not present on the surface of the protein but is embedded inside of the protein, and the glycation site is present at a place where it is difficult for an antibody to bind to the glycation site. Therefore, in order to allow an antibody that recognizes the relevant epitope site to react efficiently, a technology for exposing the relevant epitope site to the surface is being developed (Patent Document 1).
As the technique for epitope exposure, there are known techniques for treating a glycated hemoglobin-containing sample by using each of guanidine, thiocyanic acid, or lithium thiocyanate alone, by combining thiocyanic acid with an oxidizing agent such as ferricyanide, and by an ionic surfactant, a nonionic surfactant or the like (Patent Documents 1 to 6).